1. Field of the Invention
This invention relates to a solid-state light emitting device, more particularly to a light emitting diode.
2. Description of the Related Art
Referring to FIG. 1, a conventional vertically structured light emitting diode (LED) 1 includes an epitaxial layer 11 that generates light when electricity is supplied, a reflective layer 12 that is formed on a bottom surface 116 of the epitaxial layer 11 to reflect light, an electrode 13 that is disposed on a top surface 115 of the epitaxial layer 11, and a substrate 14 that is connected to the reflective layer 12.
The epitaxial layer 11 is formed by epitaxying a gallium nitride series semiconductor material on an epitaxial substrate (not shown), and includes p-type and n-type cladding layers 111,112 that are formed via doping, and an active layer 113 that is formed between the p-type and n-type cladding layers 111,112. A band gap exists between the p-type and n-type cladding layers 111,112. When electricity is supplied to the epitaxial layer 11, recombination of electron-hole pairs occurs in the active layer 113, thereby releasing energy in a form of light. The top surface 115 of the epitaxial layer 11 is roughened so as to prevent full reflection of the light produced from the epitaxial layer 11. As a result, the light can be directly emitted out of the LED 1 to the utmost through the top surface 115, thereby enhancing light emitting efficiency of the LED 1.
The reflective layer 12 is made of a material that has high reflectivity. Generally, the reflective layer 12 is made of a metal that has high reflection coefficient and is disposed in ohmic contact with the epitaxial layer 11 for electrical conduction. When the light generated from the epitaxial layer 11 is transmitted to the bottom surface 16G, the reflective layer 12 is capable of reflecting the light back to the top surface 115, thereby emitting the light out of the LED 1 and improving the light emitting efficiency of the same.
The substrate 14 is made of a thermally and electrically conductive material that is normally an alloy or a metal, is able to support the reflective layer 12 and the epitaxial layer 11, and serves as another electrode relative to the electrode 13. Furthermore, when the electricity is supplied to the epitaxial layer 11, the substrate 14 is capable of dissipating waste heat that is generated by the epitaxial layer 11 and that is subsequently transferred to the reflective layer 12. Therefore, the epitaxial layer 11 in use is maintained at a temperature that is unable to influence radiative recombination efficiency.
When the electricity is supplied to the epitaxial layer 11 by virtue of the electrode 13 and the substrate 14, current passes through the p-type and n-type cladding layers 111,112, and the active layer 113. Consequently, the light is generated due to the recombination of the electron-hole pairs. One portion of the light is directly transmitted to the top surface 115 and is able to pass through the top surface 115 with a higher chance due to roughness of the same. Another portion of the light is transmitted to the bottom surface 116, is reflected to the top surface 115 via the reflective layer 12, and can pass through the top surface 115 with a greater possibility on account of the roughness of the same as well.
During production of the LED 1, the reflective layer 12 is coated on the bottom surface 116 of the epitaxial layer 11 that is still connected to the epitaxial substrate (not shown), and is bonded to the epitaxial layer 11 by virtue of a wafer bonding process. The substrate 14 is bonded to the reflective layer 12 through wafer bonding as well. Afterward, the epitaxial substrate (not shown) is removed from the n-type cladding layer 112 so as to expose and roughen the top surface 115 of the epitaxial layer 11.
However, bonding between the epitaxial layer 11 and the reflective layer 12, and between the reflective layer 12 and the substrate 14 is not strong enough since only wafer bonding is conducted, thereby lowering a production yield of the LED 1. Furthermore, the reflective layer 12 is bonded to the epitaxial layer 11 and is disposed in ohmic contact with the same through the wafer bonding process performed at a temperature that ranges from 200° C. to 400° C. Thus, quality of the reflective layer 12 may be degraded on account of the high temperature (200° C. to 400° C.) such that reflectivity of the reflective layer 12 may be influenced. The light emitting efficiency of the LED 1 may be further lowered.
For increasing the production yield of the LED 1, the bottom surface 116 of the epitaxial layer 11 is required to be sufficiently flat so as to be tightly bonded to the reflective layer 12. Consequently, only one surface (i.e., the top surface 115) of the epitaxial layer 11 is roughened. External quantum efficiency of the LED 1 needs to be improved further.